Screening for colorectal cancer (CRC) by sigmoidoscopy results in the identification and removal of pre-cancerous lesions (adenomatous polyps), thus preventing incident cases of cancer. Current guidelines recommend screening for average-risk adults aged 50 and above every 3 to 5 years. There is little evidence supporting such an interval, however, and in fact there is some evidence suggesting that this interval is unnecessarily frequent. The objective of the proposed research is to determine an appropriate interval for screening for CRC by sigmoidoscopy. The Colorectal Cancer Prevention (CoCaP) program of Kaiser Permanente of Northern California (KPNC) has routinely entered results of their sigmoidoscopy screening program into a computer database. Using data from this unique resource, a cohort of approximately 100,000 individuals aged 50 and older who are at low risk for CRC, and who screened negative for adenomatous polyps at a baseline examination between 1994 and 1996, will be identified. Information regarding screening procedures, medical history, and demographic variables is available in electronic files, and incident cases of CRC will be identified using a population registry. Incidence rates of CRC in this population will then be calculated for successive one-year intervals following screening; all rates will be adjusted for age by direct standardization. Relative risks of CRC for later time periods following screening (as compared to the first year post-screen) will then be used to determine the screening interval that is associated with the greatest risk reduction, while maximizing the amount of time between successive screening examinations. Similar methods will be used to suggest a screening interval for a second cohort of about 15,000 individuals in the CoCaP program who had adenomatous polyps diagnosed and treated at baseline. This will provide useful information regarding screening interval in screen-positive individuals. This study will provide a rational basis to address the question of appropriate screening interval for CRC by sigmoidoscopy. Such information will allow for the reduction in CRC incidence and mortality, while minimizing the costs associated with a sigmoidoscopy screening program.